The present invention relates to fuel cell and microfluidic technology. In particular, the invention relates to systems and methods of controlling reactant fluids and pumps in micro fuel cell systems.
A fuel cell electrochemically combines hydrogen and oxygen to generate electrical energy. Commercially available fuel cell systems are still restricted to large-scale applications, such as industrial size generators for electrical power back up. Consumer electronics devices and other portable electrical power applications currently rely on lithium ion and similar battery technologies. Portable fuel cell systems offer extended usage times over batteries and would be desirable, but are not yet available.
The air readily provides oxygen; hydrogen requires a dedicated source. A portable storage device offers a replenishable hydrogen supply, and may include an outlet that detachably couples to the fuel cell system and allows the storage device to be replaced when depleted. The hydrogen supply may include a direct hydrogen supply or a ‘reformed’ hydrogen supply. A direct hydrogen supply employs a pure source, such as compressed hydrogen in a pressurized container, or a solid-hydrogen storage system, such as a metal-based hydrogen storage device. A reformed fuel cell system processes a hydrogen fuel source to produce hydrogen. The fuel source acts as a carrier for hydrogen, is manipulated to separate hydrogen, and may include a hydrocarbon fuel, hydrogen bearing fuel stream, or other hydrogen fuel source such as ammonia. Liquid fuel sources offer high energy densities and the ability to be readily stored and shipped.
One or more pumps move reactants into the fuel cell system. Portable and micro fuel cell systems use low flow rates, typically less than 5 milliliters per minute of methanol based fuels for example. Such low flow complicates accurate control—yet the fuel cell system imposes tight demands on hydrogen supply. At the least, the system must ensure that the hydrogen supply flow rate satisfies power generation in the fuel cell to meet electrical demand. The flow should also maintain correct stoichiometries for fuel processing in a reformed system; underflow may lead to an individual cell or two “going negative”, meaning that it can no longer sustain a reaction rate commensurate with the rest of the cells in a stack. Under these conditions, one or more cells in the stack may be damaged and need replacement before the stack operates properly again.
Commercially available low flow rate pumps do not provide suitable accuracy for portable fuel cell systems. Based on the foregoing, alternate techniques for reactant supply and fluid control in micro fuel cell systems are needed.